Revamp AbstractInfinispanTest thread handling to use pooling

Our base AbstractInfinispanTest class forks a new thread every time fork is called. It would provide better performance and memory usage if instead we used a pool for threads spawned in this way in the same the test.

Also we have support for running tests concurrently which currently don't work properly due to the usage of CountDownLatch which isn't quite sufficient for what it was trying to achieve. The usage of a CyclicBarrier would be more appropriate.